Is cellulose too heavy, and will it crack my plaster ceiling?

No. Cellulose is pumped in so it spreads to a light, even density right across the ceiling joists; it isn't a point load dumped in one spot. The product's stated density works out to roughly 3.5 kg per square metre at 100 mm thickness, which a plasterboard ceiling carries comfortably. Think of it as a thin, even blanket of fluffed-up paper laid across the whole ceiling, not a pile of bricks sitting on one sheet. That even spread is exactly the point: load is shared across every joist and every sheet, which is how a plasterboard ceiling is built to carry a blanket of insulation in the first place. The weight that actually cracks ceilings isn't insulation, it's water from a roof leak, which is far heavier and concentrates in one sagging patch.

At our standard ceiling depth it's only a few kilograms over each square metre. The product's stated density comes out to about 3.5 kg per square metre at 100 mm of installed thickness. Roughly the weight of a couple of bags of sugar spread over a whole square metre of ceiling, not stacked in one place. Spread that thin and even across the joists and it's a load a plasterboard ceiling is built to carry without complaint. For a sense of scale, the Australian Government's yourhome guide notes that bulk insulation works by trapping air, so it's mostly air by volume and light for its bulk. It's the even spreading, not the raw kilos, that keeps everything well within what your ceiling handles every day.

Not when it's installed to the right density. Loose-fill does need to be laid to a specified density so it holds its thickness; the CSIRO notes loose-fill materials can settle over time and that allowance for this should be made during installation, which is exactly why we pump to the product's stated density rather than under-filling. Installed properly, the weight stays light and evenly shared across every joist, so there's nothing concentrated pulling down on a single sheet of plasterboard. The jobs where a ceiling sags are almost never about insulation weight; they're a roof leak soaking the plaster, a ceiling that was already loose at the fixings, or water pooling on top. Fixing leaks and checking the ceiling fixings matters far more to your plaster than the few kilos of cellulose sitting up there.

By a long way. Water weighs about a kilogram per litre, and a leak doesn't spread out evenly the way pumped cellulose does; it runs to the lowest point and soaks into one patch of plasterboard, which is what makes ceilings sag, stain and eventually let go. A single soaked area can carry many times the weight of the dry insulation that was sitting there, all concentrated in one spot rather than shared across the joists. That's why we'd always tell you to chase a roof leak before you ever worry about the weight of your insulation. The dry, even blanket of cellulose isn't the threat to your plaster, the water getting past your roof is. Get the roof watertight and the few kilos of insulation per square metre simply aren't a structural concern.